<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        html,body {
            height: 100%;
        }
        div.container {
            height: 100%;
            background: #dddddd;
            display: grid;
        }
        div.item {
            background: #505050;
            border: 1px solid #fff;
            font-size: 16px;
            text-align: center;
            color: #fff;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="item"></div>
    </div>
</body>
</html>